The CD74 antigen is an epitope of the major histocompatibility complex (MHC) class II antigen invariant chain, Ii, present on the cell surface and taken up in large amounts of up to 8×106 molecules per cell per day (Hansen et al., 1996, Biochem. J., 320: 293-300). CD74 is present on the cell surface of B-lymphocytes, monocytes and histiocytes, human B-lymphoma cell lines, melanomas, T-cell lymphomas and a variety of other tumor cell types. (Hansen et al., 1996, Biochem. J., 320: 293-300) CD74 associates with α/β chain MHC II heterodimers to form MHC II αβIi complexes that are involved in antigen processing and presentation to T cells (Dixon et al., 2006, Biochemistry 45:5228-34; Loss et al., 1993, J Immunol 150:3187-97; Cresswell et al, 1996; Cell 84:505-7).
CD74 plays an important role in cell proliferation and survival. Binding of the CD74 ligand, macrophage migration inhibitory factor (MIF), to CD74 activates the MAP kinase cascade and promotes cell proliferation (Leng et al., 2003, J Exp Med 197:1467-76). Binding of MIF to CD74 also enhances cell survival through activation of NF-κB and Bcl-2 (Lantner et al., 2007, Blood 110:4303-11). MIF is a cytokine involved in the regulation of inflammation as well as in cell proliferation and survival of many different types of cells, including cancer and autoimmune disease cells (see, e.g., Leng et al., 2003, J Exp Med 197:1467-76; Bernhagen et al. 2007, Nature Med 12:587-96). CD74 acts in conjunction with CD44 to mediate the intracellular effects of MIF (Id.). MIF is known to suppress the action of the tumor-suppressor gene p53, resulting in promotion of cell growth and inhibition of apoptosis (Meyer-Siegler et al., 2006, J Immunol 177:8730-39). Numerous studies have demonstrated that reducing tumor MIF results in decreased cell proliferation, induction of apoptosis and reduction in the synthesis and secretion of other growth factors (Id.). Deactivation of MIF by antibodies or inhibition of MIF binding to CD74 has been shown to attenuate tumor growth and angiogenesis (Cournia et al., 2009, J Med Chem 53:416-24). MIF is also implicated in various inflammatory and autoimmune diseases, such as rheumatoid arthritis, atherosclerosis, asthma and systemic lupus (Id.)
In addition to inducing NF-κB activation, binding of MIF to CD74 up-regulates TAp63 expression, resulting in IL-8 secretion which in turn promotes cell survival (Shachar & Haran, 2011, Leuk Lymphoma 52:1446-54). TAp63 binds to the Bcl-2 promoter and induces the transcription of Bcl-2 mRNA and production of the Bcl-2 anti-apoptotic protein, which enhances cell survival (Id.). Thus, the MIF/CD74/NF-κB/Tap63 axis defines a novel anti-apoptotic pathway in mature B cells, shaping both the B cell repertoire and the immune response (Id.). Blocking of CD74 with anti-CD74 antibodies results in dramatic down-regulation of Bcl-2 expression, and augmentation of apoptosis (Id.). It now appears that the broad effects of anti-CD74 antibodies in inducing apoptosis of CD74+ cells in cancer and autoimmune disease are mediated at least in part through inhibition of MIF's promotion of cell proliferation and survival.
Antibodies against CD74 that have been reported to show efficacy against CD74-associated diseases include the anti-CD74 hLL1 antibody (milatuzumab) (Berkova et al., Expert Opin. Investig. Drugs 19:141-49; Burton et al., 2004, Clin Cancer Res 10:6605-11; Chang et al., 2005, Blood 106:4308-14; Griffiths et al., 2003, Clin Cancer Res 9:6567-71; Stein et al., 2007, Clin Cancer Res 13:5556s-63s; Stein et al., 2010, Blood 115:5180-90). However, despite the efficacy of such antibodies against cancer, some types of CD74 expressing cancers have been reported to be resistant to antibody therapy (see, e.g., Stein et al., 2010, Blood 115:5180-90). A need exists for more effective methods and compositions for therapeutic use of anti-CD74 antibodies.
Fingolimod (FTY720) is a synthetic analog of sphingosine that was developed as an immunosuppressive agent (Mandala et al., 2002, Science 296:346-349; Tedesco-Silva et al., 2005, Transplantation 79:1553-1560). Based on the results of a recent phase III clinical trial, FTY720 has been approved by the FDA as the first oral agent to treat relapsed multiple sclerosis (Cohen et al., 2010, N Engl J Med 362:402-41). The studies reported in the working Examples below demonstrate that FTY720 increases CD74 expression and sensitizes CD74 positive tumors, such as mantle cell lymphoma, to cell death mediated by anti-CD74 antibodies.